sparrow: cytogenetic effects of ionizing radiations 103 



k. Portion of the cell irradiated. — Microbeam irradiation of cells 

 has clearly illustrated that there are wide differences in sensitivity of 

 different portions of cells (205). For instance, the nucleus is much 

 more sensitive than the cytoplasm and for ultraviolet irradiation, at 

 least, the nucleolus is an extremely sensitive spot in the nucleus (48). 



1. Genotype. — Genetic differences as small as single gene differ- 

 ences can cause significant changes in sensitivity (2). Different varie- 

 ties of a single species also may show different sensitivities (114, 152, 

 202). 



2. Morphological organization and stage of development 



a. Type of cell or tissue. — In plants, as in animals, the type of 

 cell or tissue in question may have a considerable influence on the 

 radiosensitivity (8). For instance, meiotic cells are usually much 

 more sensitive than somatic cells (138, 169). Shoot and scutellum are 

 also less sensitive than roots in barley during early developmental 

 stages (104). 



b. Stage of differentiation and age of plant. — There is a consid- 

 erable amount of evidence which indicates that the tolerance of a 

 plant, as well as the number of mutations or chromosome aberrations 

 produced, may vary with the stage of differentiation of the seeds, 

 seedling, or plant (11, 14, 103, 104, 136). 



c. Portion or amount of plant irradiated. — The radiosensitivity 

 of a whole plant is greater than the sensitivity of a small area or 

 volume of a plant (58, 102, 172). 



3. Physiological conditions 



The radiosensitivity is influenced by various physiological states 

 of the cell, tissue, or plant. Space does not allow a detailed discussion 

 of these, but they are summarized in Table 8. Of the various factors 

 listed, water content of spores and seeds has been most extensively 

 studied and is known to have a large effect on radiosensitivity and on 

 yield of chromosome aberrations and mutations. Recent studies have 

 shown that the water content during post-irradiation storage of seeds 

 is also of great importance (28). It is therefore a factor which should 

 be held constant whenever possible during radiation experiments. 



The other factors listed in Table 6 will not be mentioned, except 

 to point out that it is becoming increasingly clear that the yield of 

 mutations is dependent upon the time of exposure with regard to 



